(1) Technical Field
The present invention relates to a castable, aerogel-based, ultra-low thermal conductivity opacified insulation for suppressing sublimation. More specifically, the present invention relates to an aerogel opacified with various opacifying or reflecting constituents to suppress sublimation and provide thermal insulation in thermoelectric modules.
(2) Background
Thermoelectric devices are attractive options for the generation of electricity and refrigeration because of their high reliable, silent, vibration-free operation; lack of compressed gases, chemicals, or other consumables; and complete scalability. Thermoelectric materials have been employed in space to power the Apollo, Viking, Pioneer and Voyager space missions, and are currently used in automotive seat cover coolers, in portable refrigerators that plug into an automobile's cigarette lighter, and in chemical and nuclear generators in artic regions and space probes.
Thermoelectric devices work by naturally generating a temperature gradient in the presence of an electromotive force (emf); conversely they produce an emf in a temperature gradient. While all materials except superconductors posses some thermoelectric character, only a few materials are efficient enough to generate interest. These include the lead, bismuth, and antimony chalcogenides, skutterudites (such as cobalt triantimonide), bismuth antimony, silicon germanium, boron carbides, and more complex compounds and alloys based on these materials.
One example of a thermoelectric device is a thermoelectric refrigerator. A thermoelectric refrigerator connects two or more pieces of thermoelectric material to a voltage source. One skilled in the art will appreciate that a generator can be made from the same device if the voltage source is replaced by a load (i.e., a battery charger). Nearly all thermoelectric devices use two different types of materials, one “n-type” and the other “p-type.” These materials must be connected so that they are electrically in series, but thermally in parallel.
A specific example of a thermoelectric device is shown in FIG. 1 (Prior art). In this example, the thermoelectric generators/coolers 100 employ elements or legs 102 with high aspect ratios. To efficiently generate power or cool, the legs should be insulated with insulation 104 so that heat flows through the legs rather than being radiated laterally outward 106.
One drawback of the prior art is degradation of the thermoelectric material by sublimation. Sublimation is a degradation mechanism, which can rapidly diminish performance in thermoelectric power generation. It has been previously shown that sublimation of antimony (Sb) from advanced, skutterudite (SKD) thermoelectric materials (such as CoSb3 and CeFe30.5Co0.5Sb12) degrades device performance. It has also been shown that the sublimation of Sb could be suppressed by the application of robust, micron scale coatings. These coatings consisted of thin metal foils of titanium or molybdenum. Although the films were thin enough to minimize thermal and electrical shorting, which can potentially diminish performance, coatings that are both electrically and thermally insulating are preferred.
Thus, what is needed is a system and method that allows for reduction in sublimation for thermoelectric devices to extend the life and durability of these devices.
Aerogel is a silicon-based solid with a porous, sponge-like structure in which 99.8 percent of the volume is empty space. In comparison to glass, also a silicon-based solid, aerogel is 1,000 times less dense. Additionally, aerogel has extreme microporosity on a micron sale. It is composed of individual features only a few nanometers in size. These are linked in a highly porous dendritic-like structure.
Aerogel has properties such as low thermal conductivity, low refractive index and low sound speed. Aerogel is made by high temperature and pressure-critical drying of a gel composed of colloidal silica structural units filled with solvents. Aerogel is available from Jet Propulsion Laboratory (Pasadena, Calif.).